Male porn stars and men outside the adult industry are completely different sexually. Male porn stars have a special ability to do what they do. Most men aren't capable of cumming on demand, getting a boner on demand, or holding off on cumming like men in porn.
The trick is to keep switching positions. if you're in a position and going at it and feel like you're getting close, slow down or stop, and then switch positions. Or if you're to the point where even changing up your moves won't hold you off, you can completely stop and just go down on the girl for a bit until your almost-orgasm completely fades away. Then get back to business.
And of course like others have mentioned, try masturbating more frequently. The more you play with yourself, the more accustomed you become to the way you get yourself off rather than the way sex gets you off (this doesn't apply to everyone, just some- I'm one of those included), and if you masturbate the day of, you'll likely be able to hold off during sex for at least a little longer.
Also try letting the girl take control. If you're doing doggie or missionary, try cowgirl instead, that way instead of doing what you know gets you off, the girl is in control and is doing what SHE likes instead.
Alcohol helps too
If none of these tips work, then you just need time, patience and experience. If you're not that experienced, then of course you aren't going to last long, because sex if likely one of the most amazing things in existence to you right now. With more experience, you will gain a higher tolerance.
If you are very experienced (so... meaning your cumulative number is in the double digits or you've been sleeping with the same girl(s) for years), and none of these tips work in the slightest, then you my friend have something called Premature Ejaculation and will need to see a doctor for some real medical assistance. Or be forever unsatisfied.